1. Field of the Invention
This invention relates to improvements in electrophotographic toners and, more particularly, to an electrophotographic toner whose practical performance is improved by introducing cinnamic acid into its constituent resin.
2. Description of the Prior Art
Conventionally, there are a number of methods for rendering visible an electrostatic latent image formed by electrophotographic or electrostatographic techniques. In commercial applications are widely used the cascade process in which a mixture of electrically insulating toner particles and larger carrier particles (typically, glass beads) is rolled or cascaded across the image-bearing surface and the magnetic brush process in which magnetic carrier particles (typically, powdered iron) are manipulated by magnetic force. These processes are both characterized in that the toner particles are triboelectrically charged by contact with the carrier particles and then deposited on the charged portions of the latent image by electrostatic attraction.
It is well-known that the developing toners used in these processes usually comprise a natural or synthetic thermoplastic resin having added thereto a pigment (for example, carbon black) and a dyestuff as a charge sign controlling agent. It is also well-known that the natural or synthetic thermoplastic resins useful as the principal constituent of prior art developing toners include polystyrene, polyester resin, polymethacrylate resin, polyvinyl chloride, xylene resin, polyamide resin, rosin, ester gum, shellac, etc.
However, when toners comprising any of the foregoing resins are triboelectrically charged, in the absence of a charge sign controlling agent, especially by the cascade or magnetic brush process, the magnitude of positive or negative charge so produced is not sufficient to provide a highly dense and distinct image. As is well-known, such a toner can be strongly charged by the addition of a charge sign controlling agent. By way of example, an oil-soluble azo dye containing a complex compound of chromium is added in order to allow the toner to produce a strong negative charge and a basic dye is added in order to allow the toner to produce a strong positive charge. Nevertheless, since it is generally difficult to disperse such a dyestuff evenly throughout a thermoplastic resin, uniform negative or positive charges are hardly produced on the resulting toner particles. Accordingly, toners containing such a dyestuff tend to have many disadvantages including poor adhesion to copying paper, fogging of the developed image (that is, the deposition of the toner on the uncharged or background portions of the latent image during development), low long-term stability (that is, change in triboelectric charging properties of the toner during its long-term, repeated use in a copying machine), and nonuniform image density.